feat: core SPI for contrib leaf scans (CometScanWithPlanData) [Delta contrib split, part 1]

[schenksj]

Part 1 of the Delta Lake contrib PR breakup. The native Delta scan work (delta-kernel-rs, Iceberg-style contrib) was first posted as a single ~27k-line tracking PR, #4366, which is impractical to review as one unit. This is the first of a sequence of small, independently-reviewable, independently-mergeable PRs that reconstruct that work. The full sequence and its dependency graph live in #4366.

This first slice touches core only. It adds a small extension contract that lets out-of-tree Comet contrib leaf scans (Delta now... others later?) take part in native planning without core holding a compile-time reference to them. This is the same "the edge keeps the source-specific code" shape Iceberg already uses. It ships no Delta code and is inert on default builds.

Changes

• trait CometScanWithPlanData — sourceKey / commonData / perPartitionData, plus optional dynamicPruningFilters / withDynamicPruningFilters (for scans whose DPP filters live in a @transient field that TreeNode.makeCopy cannot carry, Epic: CometNativeScan improvements (per-partition serde, cleanup, DPP, AQE DPP, V2 operator) #3510). CometNativeScanExec mixes it in.
• foreachUntilCometInput now matches case _: CometLeafExec. This is a strict superset of the previous fixed scan list: the three leaf scans it replaces (CometNativeScanExec, CometIcebergNativeScanExec, CometCsvNativeScanExec) are exactly the classes that extend CometLeafExec.
• PlanDataInjector.findAllPlanData collects per-partition planning data via the trait instead of a hardcoded CometNativeScanExec match.
• PlanDataInjector registry gains one reflective DeltaPlanDataInjector$ slot, appended to the existing injectorsByKind registry (perf: O(1) PlanDataInjector lookup by op kind #4535) only when a contrib bundled the class (-Pcontrib-delta). Default builds get ClassNotFoundException -> None and an unchanged registry. A class that is present but fails to bind (a misbuilt contrib jar) is logged, not silently swallowed.
• CometPlanAdaptiveDynamicPruningFilters rewrites AQE DPP filters in place for trait scans whose filters cannot survive makeCopy.

What this part deliberately does NOT do yet

• No perPartitionFilePaths on the trait. That member only feeds FAILED_READ_FILE error conversion and lands in a later part, after feat: surface native parquet read failures as FAILED_READ_FILE #4536 (now merged).
• No Delta code. There is no DeltaPlanDataInjector on the classpath yet, so the reflective slot resolves to nothing. This part is inert.

Why it is safe on default builds

With no contrib on the classpath the change is behavior-preserving. The leaf match is a proven superset of the old enumeration. The trait match catches the same CometNativeScanExec and still drives its subquery resolution. The reflective slot resolves to None. And the new DPP arm never fires because CometNativeScanExec leaves dynamicPruningFilters empty.

Verification

• CometScanWithPlanDataSuite (new): trait-contract defaults plus reflective-slot graceful absence. 2/2.
• CometJoinSuite (native scan fusion and the DPP path): 28/28.
• spotless and scalastyle: clean.
• No native changes in this part.

Roadmap

This is part 1 of the breakup. Subsequent parts add the build gate and inert wiring, the Rust planning and read path, the Scala claim/decline and execution, Change Data Feed reads, the test battery, and docs. Each later part is gated behind -Pcontrib-delta, so every intermediate state on main is safe for default builds. Tracking umbrella: #4366.

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🤖 AI disclosure: this PR was prepared with assistance from Claude Code (Claude Opus 4.8), under the submitter's review and direction.